1. Technical Field
The present invention relates to an AM receiving circuit that compensates sound quality of an audio signal depending on electric field intensity of a broadcast-wave signal.
2. Description of the Related Art
An AM (amplitude modulation) mode is a method of transmitting desired broadcast-wave signals (modulated signals) overlapped with amplitude of a frequency (carrier) that can be emitted from a broadcast station and is mainly used in middle-wave radio (526.5 to 1606.5 kHz). Since the electric wave in the frequency band for the middle-wave broadcast is propagated not only as surface wave but also as space wave that is reflected, especially at night, by the ionized layer (E layer) near 100 km above the ground, a wide service area can be ensured and a stable service can be supplied to mobile objects such as automobiles.
An AM receiving circuits receiving AM signals generally uses a superheterodyne detection mode. The superheterodyne detection mode is a mode of combining a signal from a broadcast station with a signal from an oscillation (local oscillation) circuit built into a receiving apparatus to detect and convert this beat into an intermediate frequency for amplification and demodulation and is characterized in that high amplification gain can be easily acquired and that cross talk can be easily prevented. Although the receiving circuit receiving AM signals must include a band-pass filter that allows passage of a frequency of a desired broadcast wave, it is very difficult to continuously change a center frequency without changing band property of a filter, the superheterodyne detection mode employs a method of changing a local oscillation frequency and allowing passage of only an intermediate frequency converted to a certain frequency.
However, in the AM receiving circuit, the AM detection level may fluctuate depending on electric field intensity of broadcast wave signals received by an antenna and cross talk may be easily generated as a result.
Therefore, a method is proposed to constrain the generation of cross talk by providing an AGC circuit for setting gains in a front end unit that processes a received RF (Radio Frequency) signal and an IF unit that processes an IF (Intermediate Frequency) signal and by changing the gains with the operation of the AGC circuit depending on the electric field intensity of the broadcast wave signals (see, e.g., patent document 1). This can constrain output fluctuations of the audio signal associated with fluctuations of the electric field intensity of the broadcast wave signals.
However, when the broadcast wave signal received by the antenna has the weak electric field intensity, the AGC circuit operates so as to increase the gains. In this case, with regard to the audio signal output as sound from a speaker of the AM receiver, a rate of noise generated by amplifiers, etc., is increased as the electric field intensity weakens in the broadcast wave signal received by the antenna. That is, the S/N ratio (Signal to Noise ratio) of the audio signal is deteriorated.
Patent document 1: Japanese Patent Application Laid-Open Publication No. 1995-22975.
When a broadcast station desired to be received (desired station) has an interfering station with a frequency near the broadcast wave frequency of the desired station, if the desired station is selected by the AM receiver, the broadcast wave signal of the desired station is affected by the frequency of the interfering station. In this case, the AGC circuit operates so as to reduce the reception levels of signals from the desired station and the interfering station and performs adjustment to reduce the gains of the front end unit and the IF unit. As a result, the desired station to be selected is suppressed by the effect of the interfering station.
To prevent the suppression, the gain of the AGC circuit can be increased. However, if the gain of the AGC circuit is simply increased, it is problematic that the saturation SN of the audio signal is deteriorated due to the constraint of the dynamic range of the IF unit.
Therefore, the object of the present invention is to provide an AM receiving circuit which improves suppression characteristics.